Fourth Layer of Sole

Introduction

Introduction to the Fourth Layer of the Sole

The fourth layer of the sole is the deepest muscular layer of the foot, lying directly on the metatarsal bones and interosseous membrane. It consists of the dorsal and plantar interossei muscles (seven in total) and the terminal portions of two extrinsic tendons: the tibialis posterior and the fibularis (peroneus) longus. This layer is responsible for fine movements of the toes, particularly abduction and adduction, and plays a crucial role in maintaining the transverse and longitudinal arches. The interossei are located between the metatarsals, with the dorsal muscles (4) abducting the toes and the plantar muscles (3) adducting them, using the second toe as the reference axis. The deep plantar arch and the deep branch of the lateral plantar nerve lie between the third and fourth layers, supplying these muscles. The fourth layer is clinically significant in deformities such as claw toes, intermetatarsal neuromas, and in the approach to the metatarsal bones for surgery.

Study

1. Contents of the Fourth Layer

The deepest layer of the sole contains two groups of intrinsic muscles (the interossei) and the insertions of two extrinsic muscles: Tibialis posterior tendon and Fibularis (peroneus) longus tendon. The interossei are arranged with reference to the second toe, which is the axis of the foot. The dorsal interossei (4) abduct the toes, while the plantar interossei (3) adduct the toes. All interossei are innervated by the deep branch of the lateral plantar nerve (S2, S3) and supplied by the deep plantar arch. The two extrinsic tendons insert onto the plantar surfaces of the tarsal and metatarsal bones, supporting the arches and contributing to inversion and eversion.

2. Dorsal Interossei

There are four dorsal interossei, numbered 1 to 4 from medial to lateral. They are bipennate muscles lying in the intermetatarsal spaces. Origin: each arises by two heads from the adjacent sides of the metatarsal bones that form its space. Insertion: the first (medial) inserts into the medial side of the proximal phalanx of the second toe. The second, third, and fourth insert into the lateral sides of the proximal phalanges of the second, third, and fourth toes, respectively. Action: they abduct the toes away from the second toe axis. The first dorsal interosseus abducts the second toe medially (toward the great toe), while the second, third, and fourth abduct the second, third, and fourth toes laterally. They also assist in flexing the metatarsophalangeal joints and extending the interphalangeal joints via their insertion into the dorsal digital expansions.

Thus, the dorsal interossei are responsible for spreading the toes (abduction). The second toe receives two dorsal interossei (one on each side) as it is the axis. The great toe has its own abductor hallucis (first layer) and does not receive a dorsal interosseus; the fifth toe has abductor digiti minimi (first layer).

3. Plantar Interossei

There are three plantar interossei, numbered 1 to 3 from medial to lateral. They are unipennate muscles lying on the plantar aspect of the lateral three metatarsals. Origin: the first plantar interosseus arises from the medial side of the third metatarsal; the second from the medial side of the fourth metatarsal; the third from the medial side of the fifth metatarsal. Insertion: each inserts into the medial side of the proximal phalanx of its corresponding toe (third, fourth, and fifth toes). Action: they adduct the toes toward the second toe axis. The first plantar interosseus adducts the third toe, the second adducts the fourth toe, and the third adducts the fifth toe. Like the dorsal interossei, they also flex the MTP joints and extend the IP joints via the dorsal digital expansions.

The plantar interossei are thus the adductors of the toes. They are smaller than the dorsal interossei. The great toe and second toe do not have plantar interossei; the great toe has an adductor hallucis (third layer), and the second toe is the axis.

4. Tendons of the Fourth Layer

Two important extrinsic tendons complete the fourth layer: (1) Tendon of tibialis posterior: After passing behind the medial malleolus, it inserts primarily into the navicular tuberosity, with slips to all tarsal bones except the talus, and to the bases of the second, third, and fourth metatarsals. It is the main dynamic inverter and arch supporter. (2) Tendon of fibularis (peroneus) longus: After passing behind the lateral malleolus and crossing the sole within a tunnel formed by the long plantar ligament, it inserts into the lateral side of the base of the first metatarsal and the medial cuneiform. It everts the foot and supports the transverse and lateral arches. Both tendons lie deep to the interossei in the tarsometatarsal region.

5. Neurovascular Supply

All interossei (dorsal and plantar) are innervated by the deep branch of the lateral plantar nerve (S2, S3). This nerve runs between the third and fourth layers, accompanied by the deep plantar arch. The deep plantar arch is formed by the lateral plantar artery anastomosing with the deep plantar branch of the dorsalis pedis artery. The arch gives off plantar metatarsal arteries, which supply the interossei, and perforating branches that anastomose with the dorsal metatarsal arteries. Venous drainage is via venae comitantes into the plantar venous system.

6. Clinical Relevance

Claw toe deformity: Imbalance between the long extensors and the intrinsic interossei/lumbricals leads to hyperextension at the MTP joints and flexion at the IP joints. Weakening or loss of interossei function (e.g., in diabetic neuropathy or Charcot-Marie-Tooth disease) is a major factor. Surgical correction may involve tendon transfers to restore intrinsic function.

Intermetatarsal neuroma (Morton’s neuroma): Though the neuroma itself is a swelling of the interdigital nerve, the interossei may be involved as the nerve passes between the metatarsal heads. Tight intermetatarsal spaces or hypertrophy of the dorsal interossei can contribute to nerve compression.

Tibialis posterior dysfunction: Dysfunction or rupture of this tendon leads to progressive flatfoot deformity (pes planovalgus), as the main dynamic support of the medial longitudinal arch is lost. The tendon is assessed by the single-heel rise test.

Fibularis longus tendon pathology: Tenosynovitis or rupture can cause lateral pain and weakness of eversion and plantar flexion of the first ray, leading to a pronated foot posture.

Metatarsal fractures and compartment syndrome: Because the interossei occupy the intermetatarsal spaces, fractures of the metatarsal shafts can cause bleeding into these compartments. The fourth layer muscles are within the interosseous compartments of the foot, and compartment syndrome may require fasciotomy of the interosseous compartments.

Summary

Summary of the Fourth Layer of the Sole

The deepest layer consists of the dorsal (4) and plantar (3) interossei, plus the tibialis posterior and fibularis longus tendons. The dorsal interossei abduct the toes, the plantar interossei adduct them, all innervated by the deep branch of the lateral plantar nerve (S2, S3). The tendons support the arches and enable inversion/eversion. The deep plantar arch and nerve lie between the third and fourth layers. Clinical significance includes claw toe deformities, Morton’s neuroma, posterior tibial tendon dysfunction, and interosseous compartment syndrome.